One of the vital components of wireless equipment is a power amplifier. The power amplifier includes an amplification device and an impedance matching circuit. Conventional matching circuits satisfy matching requirements only at a certain frequency. However, recent mobile communication systems, for example, use multiple frequency bands. Therefore there is a need for power amplifiers that operate in multiple frequency bands.
One such technique is disclosed in Japanese Patent Application Laid-Open No. H11-97946. In the technique, multiple band-pass filters, each passing a corresponding signal in a superimposed input signal, are provided. Each band-pass filter transmits a signal in the single frequency band other than signals in other frequency bands. Matching circuits that operate in their respective frequency bands are also provided. An example of the technique is illustrated in FIG. 1A. In this example, two signals, a high frequency band signal and a low frequency band signal, are amplified. One end of a high band pass filter (HBPF) 4H and one end of a low band pass filter (LBPF) 4L are connected to an input terminal P2 connected to an output terminal of a given device (for example an amplification device 20). A high band matching circuit 5H is connected between the other end of the HBPF 4H and an output terminal P1H. A low band matching circuit 5L is connected between the other end of the LBPF 4L and an output terminal P1L. Loads 30H and 30L with impedance Z0 are connected to the output terminal P1H and P1L, respectively.
The high band matching circuit 5H matches the impedance at the input terminal P2 to the impedance at the output terminal P1H in the high frequency band by taking the presence of the HBPF 4H into account. The low band matching circuit 5L matches the impedance at the input terminal P2 to the impedance at the output terminal P1L in the low band by taking the presence of the LBPF 4L into account. The band-pass filter is typically made up of multiple reactance elements.
The configuration illustrated in FIG. 1A requires an increased number of filter stages in order to sufficiently attenuate out-of-band signals. However, the increased number of filter stages causes the problem of losses in passband signals.
In order to increase the degree of isolation between the high-frequency band circuit and the low-frequency band circuit, as shown by the dashed line in FIG. 1B, it may be contemplated to provide a switch SWH between the input terminal P2 and the HBPF 4H and a switch SWL between the input terminal P2 and the LBPF 4L. However, when the output impedance of the transistor that constitutes a high power amplification device 20, for example, is very small (for example less than a few ohms), a considerably high current flows through the switches and accordingly a significantly large insertion loss is caused by the on-resistances of the switches. In view of this problem, it is desirable that switches with low-on-resistances be used. However, low-on-resistance switches have a problem that the degree of isolation decreases when the switches are in the off-state. Therefore, in the state of the art, there has been no choice but accept a certain level of insertion loss caused by the on-resistance of switches and high current flowing through the switches.